Horizontal-axis wind turbines for generating electricity from rotational motion are generally comprised of one or more rotor blades each having an aerodynamic body extending outwards from a horizontal shaft that is supported by, and rotates within, a wind turbine nacelle. The nacelle is supported on a tower which extends from the ground or other surface. Wind incident on the rotor blades applies pressure causing the rotor blades to move by rotating the shaft from which they extend about the horizontal rotational axis of the shaft. The shaft is, in turn, associated with an electricity generator which, as is well-known, converts the rotational motion of the shaft into electrical current for transmission, storage and/or immediate use. Horizontal-axis wind turbines are generally very well-known and understood, though improvements in their operation to improve the efficiency of power conversion and their overall operational characteristics are desirable.
Incident wind at even low speeds can cause the rotor blades to rotate very quickly. As would be well-understood, for a given rotational velocity, the linear velocity of a rotor blade is lowest in the region of its root—the portion of the rotor blade proximate to the shaft. Similarly, the linear velocity of the rotor blade is highest in the region of its wingtip—the portion of the rotor blade distal from the shaft. Particularly at higher linear velocities, aspects of the rotor blade can generate significant aeroacoustic noise as the rotor blade rapidly “slices” through air along its rotational path. This noise can be quite uncomfortable for people and animals in the vicinity to witness. However, the noise can also be an indicator that operation is not efficient, and maximum wingtip speed can actually be limited by such inefficiencies.
Wind turbines are increasing in popularity in recent years as a means of generating renewable energy. With this growth, optimal locations for their operation have been subsequently declining, with these locations being limited. As a result, wind turbines have been placed closer and closer to communities, accordingly placing the noise that the wind turbines generate closer to people who can hear it. Complaints and resistance from neighbours of wind turbine developments can mount, particularly in respect of complaints of “hissing” or “swishing” sounds in the 1 kHz frequency range. Only recently has noise emissions become a concern for rotor blade designers, who must balance many criterion to produce the optimal rotor blade. However, since noise is a form of energy, decreasing noise emissions may also have a positive benefit to energy production, since energy will not be lost in the production of sound waves.
Noise emissions from the rotor blade either come from the tips, called tip vortex noise, or from the trailing edge near, but not at, the tip. Rotor blade noise has been found to mostly consist of trailing edge noise, and comes in two varieties—blunt trailing edge noise, or “B-TE” noise, and turbulent boundary layer trailing edge noise, or “TBL-TE” noise, with TBL-TE being the largest cause for rotor blade noise emissions. TBL-TE is caused by scattering of turbulent fluctuations within the blade boundary layer at the trailing edge, resulting in radiation of broad-frequency noise. It would be useful to enhance the structure of a rotor blade in an attempt to reduce TBL-TE rotor blade noise emissions.
Straight serrations that follow the blade suction-side contour near the trailing edge have been explored as a means for reducing the scattering of turbulent fluctuations within the blade boundary layer at the trailing edge and have been shown to reduce the total sound pressure level by 2 dB, dominated by reductions in noise at relatively low frequencies. U.S. Patent Application Publication No. 2008/0166241 to Herr et al. discloses a means of reducing the noise emissions of a rotor blade during use by employing bristles at the trailing edge of a rotor blade. According to the inventors, for reducing trailing edge related noise, shorter bristles achieve better reduction results for lower frequencies, whereas longer bristles tend to be more effective for higher frequencies. The inventors explain that a combination of bristles with significantly different outer dimensions in the same region of the blade contributes to a reduction characteristic with a higher efficiency in a broad frequency spectrum.
The radiated noise from a rotor blade is loudest for an incident pressure wave that is aligned with the edge of the rotor blade and traveling normal to that edge. As the pressure wave passes over the edge, it encounters a sudden change in acoustic impedance, resulting in the scattering of noise. The bristles can be viewed as a means of distributing this sudden change in impedance over a finite distance, thereby reducing the strength of the scattering process. However, the straight serrations also resulted in a significant increase in noise at high frequencies (>2000 Hz). Thus, one skilled in the art would recognize that the use of straight serrations requires careful placement and a careful consideration of widths and lengths in order to achieve a desired effect. In addition, turbulent inflow noise may contribute to the noise spectrum of a wind turbine at low frequencies. Thus, one skilled in the art must manage both the incoming and the out going air flows in order to achieve a desired effect.